Should I take my son to the ER? Fever, vomiting, stomach pain...?

He is sick, he is dehydrated, and he needs to see a doctor or nurse practitioner. Most walk-in clinics won't see a child this sick. Go to the ER now. They will not admit him unless he needs to be admitted. The sooner you go, the sooner you will get back to your vacation. Stop typing long narratives on the computer and go. I'm not judging you, just go and get some help for your child.